Composite video signals, commonly used in video broadcasts or transmissions, contain a brightness signal (luminance, luma or Y) and a color signal (chrominance, chroma or C), where the color signal is modulated into a color sub-carrier and added to the brightness signal prior to transmission. To effectuate demodulation of the color sub-carrier upon reception, receivers for color displays include a Y/C separator to separate luminance and chrominance components from the composite video signal.
Y/C separation is typically performed with either a notch/band pass filter combination or comb filters, where each filter separates portions of a composite video signal more effectively. For instance, a band pass filter used in conjunction with a notch or comb filter can separate the chrominance component from a composite signal. However, in addition to passing the chrominance signal, a notch/band-pass filter will pass high frequency luminance energy in the chrominance pass band. Therefore, some luminance appears as cross-color in a displayed picture. For instance, a vertical striped pattern on a displayed article of clothing can have a spatial frequency creating a high frequency luminance component, which appears as cross-color in a separated chrominance signal. This cross-color usually appears in a video picture as a colored rainbow superimposed on the reduced-amplitude striped pattern.
Although the notch/band pass filter combination has drawbacks, it is useful under certain conditions. For example, the filter combination is effective in picture regions lacking high luminance frequencies in the horizontal direction while containing high frequency chroma information in the vertical direction. In these regions, the comb filter described below degrades pictures with cross luma.
Comb filtering is another technique for Y/C separation. Comb filtering usually provides a considerably better component separation compared to the above-described notch/band-pass filter combination. Although conventional comb filters provide improved separation, crosstalk between the chrominance and luminance still occurs. Comb filters are therefore most effective in pictures having a flat field of color or high luminance frequencies in the horizontal direction. When spatial discontinuities in the vertical direction occur in a video picture, however, a conventional comb filter may inadequately separate chrominance and luminance components, causing undesirable artifacts in the displayed picture.
To optimize Y/C separation, many Y/C separators implement both separation techniques and adapt between the techniques responsive to the horizontal and vertical transitions of the video signal. Many approaches for adapting between the separation techniques exist. For instance, an adaptive Y/C separator may select between the outputs of the notch/band pass combination or the comb filters. When neither separation technique is optimal, however, the selection of either technique results in an undesirable picture quality. To counter this picture quality degradation adaptive Y/C separators blend the outputs of notch/band pass combination or the comb filters. Blending is typically implemented by generating a blending coefficient and combining the outputs of notch/band pass combination or the comb filters responsive to the blending coefficient. Each blending coefficient, however, is difficult to normalize, which causes the adaptive Y/C separator to not adapt when low chroma levels are present and to transition too quickly for high chroma levels.
Accordingly, there exists a need for an improved adaptive Y/C separation system and method that adapts to changes in a video signal without degrading the picture quality.